Volquez is still an unknown, Leake has to be worked around to a point, Bailey is still hurt, Wood has one big league start to his name. The rotation is far from a finished product.
Trade for Lee and your rotation is:
If they keep Leake's innings down, Volquez works out and Bailey comes back ready to go, that's a good problem to have. You sit Harang down and say "Aaron, you're getting older. You're not going to be here next year in all likelihood and you have no guarantees on the open market. Decent chance you end up on a crappy team for way less money. This is your chance to win a title. We need you in the pen more than the 4 man playoff rotation."Harang as the long man is an upgrade over Owings.
If they make the postseason and everyone is doing their jobs, someone gets left off the roster for each series. That's a call that would have to be made anyway, I'd rather have to make that call with Lee on the staff than without.